1. Field of the Invention
The present invention relates to an apparatus for optically recording and reproducing information and a method using the same.
2. Description of the Related Art
Recently, apparatus for optically recording and reproducing information have been remarkably developed in the fields of the information and communication industry as well as the audiovisual industry. Especially, an optical disc in which information, as a concavo-convex pit array, is stored in a circular substrate and light is condensed onto a micro-spot so as to reproduce the information, has been standardized and spread all over the world under the name of a compact disc (hereinafter, referred to as a "CD"). Such a system utilizes the diffraction phenomenon of light for reproducing information. That is, the presence or absence of a pit can be determined by detecting the intensity of light which is reflected from the surface of the CD, since there is a difference between the reflectivity of a pit-present portion and that of a pit-absent portion, which is caused by the diffraction of the light. According to the current CD standard, the difference in reflectivity is expected to be about 30%. A reproduced signal is represented by binary digits showing the presence or absence of the pit. The length of the pits and/or the spacings between the respective pits is determined based on the contents of the information to be stored. That is, the information is recorded as a size change of surface features of a recording medium in an in-plane direction, so that the improvement of the storage density of the CD is largely restricted by the diffraction limit of a reproduced light. Another method is proposed, in which the difference in reflectivity of 20% or less is obtained by utilizing the fact that the reflectivity changes as the phase of medium changes.
Japanese Laid-Open Patent Publication No. 4-14620 proposes an information recording and reproducing system utilizing surface plasmon resonance in order to remarkably improve reproduction sensitivity much more than the above systems presently do. According to this system, a recording medium is obtained by laminating arachidic acid cadmium salt on a silver layer, and the presence or absence of the arachidic acid cadmium salt is detected with a difference in reflectivity of 90% or more. Thus, according to this system utilizing the surface plasmon resonance, remarkably higher difference in reflectivity (90% or more) can be obtained compared with the system utilizing the diffraction phenomenon of the light ( about 30%), and the system utilizing the change of the phase of the medium (20% or less). However, this system also has the same problem of the storage density as the system utilizing the diffraction phenomena has, since it also employs the same recording method in which the information is represented by the binary digits showing the presence or absence of the arachidic acid cadminate layer, and then the information to be recorded is modulated and represented by the length of the pit and the spacing between the respective pits. That is, even if employing the system utilizing the surface plasmon resonance, the pit density is restricted by the spot diameter during the reproduction performance, and the spot diameter is determined by an optical wavelength and a numeral aperture (NA) of the condenser lens. Therefore, in order to improve the storage density, a power source having a short wavelength or a lens having a large NA is required. Therefore, this system utilizing the surface plasmon resonance can improve the reproduction sensitivity but cannot improve the storage density compared with the system utilizing the diffraction phenomena of the light.